Cake washing wash ratio

Usually, cake wash ratios of about 0.1 to 0.3 kg wash/kg sohds are possible within the normal residence time of the wash zone. This usually can displace at least 95 percent of the mother liquor and impurities. In some cases higher wash ratios or even multistage countercurrent washes are utihzed, in which case sufficient residence time via throughput reduction must be considered. 

Cake Washing Wash efficiency data are most conveniently represented by a semilog plot of percent remaining B as a function of wash ratio N as shown in Fig. 18-103. Percent remaining refers to that portion of the solute in the dewatered but unwashed cake which is left in the washed and dewatered cake. Since a cake-washing operation... 

Practical experience has shown that the most convenient and best means of expressing R is in terms of the solute concentrations in the washed cake hquid the feed liquid (or unwashed cake liquid), and the cake wash hquid. Furthermore, the wash ratio N may also be expressed either as a volume or weight ratio. 

If it were possible to obtain a perfect sing displacement wash, the frac tion remaining would be numerically equal to I minus the wash ratio. This ideal condition is represented by the maximum theoretical line as shown in Fig. 18-103. Since it represents the best that can be done, no data point should fall to the left of this cni ve. Most, but not all, cake-washing curves tend to fall along the heavy solid line shown. In the absence of ac tnal data, one may estimate washing results by using this cni ve. 

V = volume of cake wash/nnit area/cycle N = wash ratio... 

Wash water is pumped through the filter cake at a predetermined wash ratio to displace the adhering mother solution. 

Application Filter area, m2 Slurry feed characteristics % solids pH t/hr Wash ratio (wt/wt based on dry solids) Solubles recovery % Final cake moisture %... 

Experimental wash curves represented as fraction of solute remaining versus the wash ratio j (ratio of wash to void volume of cake) can be plotted semilogarithmically as in Fig. 14.11 (the solid line). No experimental point will fall on the left of the maximum theoretical curve (the dotted line), which represents perfect displacement. 

The following conditions and specifications are assumed slurry contains 40% solids by weight solute in the liquid is 2% final cake moisture is 25% wash ratio (wash volume per void volume) is 1.5 ... 

Several leaf tests should be performed for repeatability. Data collected will permit scaleup to plant scale operations. Significant data will be pounds of dry cake per square foot per hour, gallons of filtrate per square foot per hour, filtrate clarity, wash ratios, (pounds of solids/gallon of wash), residual moistures, filtermedia selection, knife advance time, precoat thickness, solids penetration into precoat, and submergence level should also be evaluated. For the optimization equation, refer to Peters and Timmerhaus, and Tiller and Crump. 

Wash ratio j a ratio equaling the total volume of wash liquid exiting a cake to the volume of void of the cake. 

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